At one time I examined a lot of 30 cal military loads by pulling them down. The match loads had neither a crimp nor neck hold. The bullets were glued in place with pitch sealant, and if you pulled one and used a Q-tip and some mineral spirits to dissolve the pitch out of the neck and off the bullet, the bullet could be easily pushed into and pulled out of the neck by hand.
The M2 ball ammo I looked at had been made the same way, but a Lee Factory Crimp type of collet crimp had been applied that significantly deformed the midsections of the bullets. You can see how they would be concerned that in combat automotive fuel or other things that could dissolve the pitch might get onto some ammo and slowly loosen the grip of the pitch on the bullet.
For handloads I've never seen a bullet fail to be held adequately by its press-fit into a resized case neck, something the old military ammo didn't have. If you have a crimp cannelure and intend to use it, you normally use a roll crimp or a taper crimp with a fairly steep angle so its pressure on the brass below it doesn't get stopped by or wind up distorting the bullet just below the cannelure. Where a taper crimp really comes into its own on a rifle cartridge is when you have a flared neck from an expander for cast bullets. Where that also helps with jacketed rifle bullets is if you get one of the Lyman M dies and set it up just to put a small step inside the case mouth. The step hold the bullet centered and upright as you start it into the seating die (no more hanging on with your thumb and index finger until the bullet tip is inside the die mouth). This also causes bullets to seat straighter into the neck which produces an actual accuracy improvement, but you need to iron out the step afterward, and just kissing it with a shallow angled taper crimp die does that nicely.
The Lee Factory Crimp die's collet style crimp indents a bullet under the crimp area. You can seat a cannelure down to meet that crimp or ignore it. The main argument has been whether this indenting helps or creates problems. The answer is, it depends. If the distortion is all pretty far forward of the bottom of the bullet bearing surface and doesn't distort the base or boattail shoulder at all, the impact of a shallow symmetrical distortion on ballistics is typically indiscernible. Bear in mind that rifling engraving marks don't bother a bullet's ballistics because they are shallower than the air boundary layer that forms over the moving bullet. A shallow crimp indentation (no deeper than rifling engraving) will be the same way. However, the deeper you apply such a crimp the greater the likelihood of the distortion becoming asymmetrical, moving the bullet's center of gravity off its longitudinal axis and causing the bullet to wobble in flight, both widening groups and reducing effective ballistic coefficient, though that latter effect wouldn't only show up at very long ranges.
As Lee claims, crimps do increase start pressure, improving powder ignition consistency, and that can result in tighter groups in some guns over some ranges. That applies to any form of crimp if it is consistent. An advantage the Lee tool has toward consistency is that small differences in case trim length have no effect on how firm its crimp is, as they would with roll and taper crimps into a cannelure.
The bottom line is you get to try this stuff out to see how it does in your gun. Note that no commercial match loads use crimps because everybody is concerned about distorting the bullets. But, unlike the sealed military ammo, the commercial match loads do have a bullet press fit into the narrowed neck holding onto the bullet with something like 30-40 lbs of force in the .223, and these all work fine in AR's, so this isn't a functional issue. For the usual handloader not hauling the ammo into a war zone, it's really just a matter of seeing if an accuracy improvement is to be had from using a crimp in your equipment with your components.
